This invention relates to a high strength, low alloy, dual phase steel having an improved combination of formability and high product strength. More particularly, this invention relates to a microalloy-free, low carbon, manganese steel characterized by a ferrite matrix microstructure comprising dispersed particles that are initially austenite, but transform to martensite during a forming operation. This invention also relates to a heat treatment for low carbon, manganese steel to form the initial ferrite-austenite microstructure.
Dual phase steel is high strength, low alloy steel that is characterized by a matrix microstructure composed of a continuous ferrite phase having a second phase distributed therein. In conventional dual phase steel, the second phase is martensite, although austenite or bainite may also be present, in which case the steel may include more than the two iron metallurgical phases strictly implied by its name. Dual phase steel is used, for example, in the manufacture of cold-formed sheet steel articles. The composite microstructure produces an advantageous combination of mechanical properties that allow the steel to be readily formed, but to develop a high formed strength. Although several factors are involved in achieving these advantageous properties, the high formability is attributed in part to the high ductility of the ferrite matrix, while the dispersed martensite reinforces the matrix to help develop the high product strength. Typically, the ferrite-martensite microstructure is produced by a heat treatment that includes in a final anneal at an intercritical temperature whereat ferrite and austenite phases coexist, followed by cooling, during which austenite transforms to create the dispersed martensite phase.
One example of a dual phase steel is referred to in the industry as GM 980X. GM 980X is a low-carbon, manganese steel that includes a small but critical vanadium addition. Although the basis of a ferrite-martensite dual phase microstructure is produced in GM 980X after an intercritical heat treatment, it has been found that a portion of the austenite does not transform during cooling, but is retained in the microstructure. Under low strain such as experienced during a forming operation, this retained austenite transforms to additional martensite. The retained austenite improves formability, at least partly by reducing yield strength and increasing plasticity. In addition, the increase in martensite during forming further reinforces the steel to thereby increase product strength.
Thus, the nonferrite constituent initially in GM 980X is mostly martensite, but includes significant retained austenite capable of strain-induced transformation during a subsequent forming operation. The formation of this initial microstructure, including the metastable retained austenite, is attributed in large part to the presence of the microalloy vanadium addition. However, vanadium, even in microalloy quantities, significantly increases steel cost. Heretofore, only minor quantities, if any, of retained austenite have been found in microalloy-free dual phase steel. For example, Koo et al, "Thermal Cycling Treatments in Microstructures for Improved Properties of Fe--0.12% C--0.15% Mn Steels", Materials Science and Engineering 24 (1976), pp. 187-198, describes a microalloy-free, low-carbon, manganese dual phase steel wherein the dispersed constituent is predominantly martensite, but includes retained austenite closely associated with the martensite. The minor proportion and martensite affiliation of this austenite indicates that any effect upon the steel properties is minimal, particularly in comparison to the martensite initially present in the steel.
It is an object of this invention to provide a microalloy-free, low alloy, low carbon, manganese steel having a dual phase microstructure wherein the dispersed constituent is predominantly retained austenite, which retained austenite cooperates with the ferrite matrix initially to improve formability, but undergoes strain-induced transformation to martensite during a forming operation to increase product strength. The high proportion, distribution and morphology of the retained austenite contribute to its particular effectiveness in improving the desired steel properties. The improved combination of high form-ability and high product strength is particularly advantageous in the manufacture of sheet steel articles and the like, but is achieved without the expense of a microalloy agent.
It is also an object of this invention to provide a heat treatment process for a microalloy-free, low carbon, manganese steel to form a ferrite-austenite dual phase microstructure. This process is applicable to steels having a conventional composition, including standard grade steels. By developing the dual phase microstructure, the process of this invention improves steel formability and product strength.